The first half of the journey, the car drove at a speed of 90 km / h, and the second half

The first half of the journey, the car drove at a speed of 90 km / h, and the second half of the journey – at a speed of 70 km / h. Find the average speed of the car along the way.

The average vehicle speed is calculated using the following formula:

Vav. = S / t, where S is the total distance traveled by the car (S = S1 + S2 = 2S1 = 2S2; S1 = S2 = 0.5S), t is the travel time (t = t1 + t2).

First half of the journey:

V1 = 90 km / h, t1 = S1 / V1 = 0.5S / 90.

Second half of the journey:

V2 = 70 km / h, t2 = S2 / V2 = 0.5S / 70.

Let’s do the calculation:

Vav. = S / (0.5S / 90 + 0.5S / 70) = 1 / (1/180 + 1/140) = 78.75 km / h.

Answer: The average vehicle speed is 78.75 km / h.